In the graphic arts, it is desirable to produce a color proof to assist a printer in correcting a set of photomasks which will be used in exposing printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired half-tone or line image, and should neither gain nor lose color. Visual examination of a color proof should reveal the following characteristics:
1. Any defects on the photomask. PA1 2. The best color rendition to be expected from press printing of the material. PA1 3. The correct gradation of all colors and whether grays are neutral. PA1 4. The need, if any, for subduing any of the colors and/or giving directions for altering the film photomask before making the printing plates. PA1 (i) laminating a presensitized, photosensitive color proofing element with heat and pressure via the thermal adhesive to a developer-resistant receiver sheet; PA1 (ii) removing the carrier support; PA1 (iii) exposing the color proofing element through a color separation negative corresponding to the pigment of the color layer to actinic radiation to cross-link the exposed areas of the color proofing element and rendering the color and barrier layers insoluble in an aqueous alkaline developing medium to create a latent image; PA1 (iv) developing the latent image with an aqueous alkaline developing medium whereby unexposed color and barrier layers are removed and exposed color and barrier layers remain; PA1 (v) laminating an additional color proofing element having a different color pigment in the photosensitive color layer to the developed first color proofing element; PA1 (vi) repeating steps (ii)-(iv) with a subsequent separation negative being in register with the developed first color proofing element; and PA1 (vii) repeating steps (v) and (ii)-(iv) in that order with additional color proofing elements as described above, each of the color proofing elements being of a different color, to provide a multicolored image on a single substrate. PA1 a) a thermal adhesive having a glass transition temperature ranging from about -10 to about 60.degree. C.; and PA1 b) a release agent selected from the group consisting of (1) copolymers having at least one vinyl polymeric segment and at least one siloxane polymeric segment; (2) copolymers having an acrylic acid ester of an alkyl-terminated alcohol wherein the terminal alkyl chain is from 12 to 24 carbon atoms in length; and (3) copolymers derived from ethylenically-unsaturated fluoroalkyl-containing monomers. PA1 a) a carrier layer; PA1 b) a color layer; and PA1 c) an adhesive layer comprising a thermal adhesive and a release agent, wherein the thermal adhesive and release agent are described above. PA1 R.sub.2 can independently be the same or different and are divalent linking groups; PA1 R.sub.3 are monovalent moieties which can independently be the same or different and are selected from the group consisting of alkyl, aryl, alkaryl, alkoxy, alkylamino, hydroxyl, hydrogen and fluoroalkyl; PA1 R.sub.4 can be independently be the same or different and are divalent linking groups; PA1 G.sub.5 are monovalent moieties which can independently be the same or different and are selected from the group consisting of alkyl, aryl, alkaryl, alkoxy, alkylamino, fluoroalkyl, hydrogen, and --ZSA; PA1 Z is a divalent linking group; PA1 A is a vinyl polymeric segment consisting essentially of polymerized free radically polymerizable monomer; PA1 G.sub.6 are monovalent moieties which can independently be the same or different and are selected from the group consisting of alkyl, aryl, alkaryl, alkoxy, alkylamino, fluoroalkyl, hydrogen, and --ZSA; PA1 G.sub.2 comprises A; PA1 G.sub.4 comprises A; PA1 x is an integer of 0-3; PA1 y is an integer of 10 or greater; and PA1 q is an integer of 0-3. PA1 A is at least one free radically polymerizable vinyl monomer; PA1 B is at least one polar monomer copolymerizable with A, the amount of B being of up to 30% of the total weight of all monomers, and PA1 C is a monomer having the general formula
Color proofing sheets for multicolored printing have heretofore been made by using a printing press proof which requires taking all the steps necessary for actual multicolor printing. Such a conventional method of color proofing has been costly and time consuming. Color proofing methods have therefore been developed to simulate the quality of press proofs.
An exemplary color proofing system is disclosed in U.S. Pat. No. 5,248,583, the disclosure of which is incorporated herein by reference. This patent provides a negative-acting color proofing photosensitive element comprising, in order (a) a carrier support, (b) a release layer which also acts as an oxygen barrier, (c) a photosensitive color layer containing a photopolymerizable multifunctional acrylate oligomer, (d) a photopolymerizable barrier layer and (e) a thermal adhesive.
A multicolored image on a single substrate can be obtained by performing the following procedure using the aforesaid photosensitive color proofing element:
A positive-acting color proofing system is disclosed in U.S. Pat. No. 4,260,673, the disclosure of which is incorporated herein by reference. This patent describes a presensitized color proofing element which includes a carrier sheet having a smooth release surface. Coated onto the release surface of the carrier sheet in a clinging engagement is a color coating of a mixture of a diazo oxide and a pigmented base-soluble resin compound. Bonded to the top of this color coating is a discrete binder layer of a mixture of a base-soluble resin and a diazo oxide. Both the binder layer and the color coating are solubilizable in a solvent developing medium upon exposure to actinic radiation. Firmly attached to the binder is a clear barrier layer which is insolubilizable in the development medium.
In order to obtain a multi-colored color proof on one substrate, the barrier layer of a first proofing element of the aforementioned construction is bonded to a substrate and the carrier sheet removed. The substrate with the presensitized proofing sheet bonded thereto is then exposed to actinic radiation through a color separation positive corresponding to the pigment of the color coating. The exposed diazo oxide and resin mixture in the binder layer and the color coating is rendered soluble to a solvent developing medium to create a latent image.
The latent image may then be developed with a solvent developing medium whereby the exposed diazo oxide and resin mixture in the binder layer and associated color coating is removed leaving the unexposed diazo oxide and resin. This process is repeated in sequence and in register so that the composite proof contains colored layers representing the magenta, cyan, yellow, and black contributions of the original subject matter.
A problem which is common to all sheet materials having a thermal adhesive on at least one surface is blocking. Blocking is a condition where separate sheets of material have become fused together to the point where the act of separating the sheets results in damage to any portion of the sheet. Such undesirable adhesion between touching sheets typically occurs under moderate pressure and at elevated temperature during storage. There is often impairment of the adhesive or the contacted surface when an attempt is made to separate members that have bonded.
Color proofing elements typically have a thermal adhesive on at least one surface and are stacked during storage. Thus, blocking can be a major problem. It has been conventional practice to add antiblocking agents to adhesive layers, and in some instances to the opposed surfaces. Typical antiblocking agents include particulates (e.g., silica or polymethylmethacrylate), and soluble organic materials that dissolve in the adhesive. Generally, the effectiveness of particulate antiblocking agents is directly proportional to the size of the particles and to the number of particles per unit area. However, large particles can be forced into previously-coated lower layers during lamination, including the color layer, thereby causing color-voids to appear in the final proof. In addition, high particulate concentrations cause haze to appear in the proofing element itself and in the final proof.
Soluble antiblocking agents often do not coat out well in certain adhesives and can solubilize important ingredients in adjacent layers. This last problem is particularly damaging in imageable layers with critical sensitometric and color balance problems such as those in prepress color proofing elements.
Another solution to the problem of blocking in color proofing elements is the use of thermal adhesives which have higher glass transition temperatures ("Tg"), i.e., greater than about 50.degree. C. However, a key requirement of the adhesive is that during lamination, the adhesive must flow into the imaged relief of the color image that it's being laminated to. Incomplete flow into the relief results in entrapped air bubbles (referred to as microbubbles). The presence of larger microbubbles detracts from the appearance of the final proof. To minimize the size of the microbubbles, the adhesive must have good melt flow properties under laminating conditions. Under typical laminating conditions, good melt flow properties will occur if the Tg of the thermal adhesive is less than 50.degree. C., preferably less than 40.degree. C. Adhesives having such low glass transition temperatures, however, are more likely to cause blocking than adhesives with higher glass transition temperatures.
Accordingly, it would be desirable to use a thermal adhesive with as low a Tg as possible while avoiding the effect of blocking between individual color proofing sheets.